99mTc-FAPI Scintigraphy in Metastasis Workup of Sinonasal Rhabdomyosarcoma; Unveiling Diffuse Bone Marrow Involvement

We present the case of a 45-year-old man diagnosed with high-grade sinonasal rhabdomyosarcoma (SNRMS) after being evaluated for a cervical mass. Five months after successfully completing chemo-radiotherapy, he developed pancytopenia and experienced progressive symptoms, including weakness and generalized bone pain, which prompted further evaluation. While an MRI of the brain and paranasal sinuses appeared unremarkable, a 99mTc-MDP scan revealed the presence of bone metastases. The increased uptake in the periarticular regions raised concerns about potential bone marrow involvement. Notably, 99mTc-FAPI-46 scintigraphy showed diffuse uptake in the bone marrow, indicating extensive infiltration. This case emphasizes the potential of FAPI imaging to enhance the assessment of disease extent in SNRMS, highlighting its role in identifying bone marrow involvement.